This invention relates generally to color television receivers, and particularly to such receivers employing automatic control of color level.
Contemporary color television receivers usually include an ACC (Automatic Color Control) circuit for automatically adjusting the level of the 3.58 MH.sub.z color signal which becomes demodulated and applied to the control electrodes of a cathode ray tube. Conventionally, such ACC operation causes the controlled level of the color signal to follow the amplitude of the 3.58 MH.sub.z reference burst signal. Thus, as the received level of the reference burst varies, the level of the color signal varies accordingly.
As is well known in the art, however, transmission inconsistencies and receiver variables sometimes produce variations in the amplitude relationship between the reference burst signal and the color signal. Consequently, the displayed image can appear as over-saturated or as nearly colorless, depending on the direction of variation between the amplitude of the color signal and the reference burst signal.
To overcome the problems associated with such variations between the amplitudes of the color signal and the burst reference signal, it has been proposed to use ACC control in conjunction with an automatic color limiter which limits the amplitude of the color signal independently of the magnitude of the burst signal. Thus, for example, when the ACC operation causes the level of the color signal to be greater than desired, the automatic color limiter limits the color signal to a predetermined maximum level. When ACC operation causes the level of the color signal to be below the predetermined maximum level, the automatic color limiter does not change the amplitude of the color signal. In this manner, undesirably high levels of the color signal generated as a result of improper ACC action are compensated for. Such a system is shown, for example, in U.S. Pat. No. 4,106,054, assigned to the assignee of the present invention.
Although the system disclosed in the above-mentioned patent performs satisfactorily, it is desirable to provide a simple and inexpensive means by which the automatic color limiter can be manually enabled and disabled according to the preference of the viewer, or automatically in response to a change in the broadcast signal.
Another limitation of many conventional color receivers is that they do not include a so-called "color killer" adjustment by which a viewer can alter the point at which color signals are reproduced. Frequently, a non-adjustable color killer quenches color when a received signal either has a color component of a predetermined small amplitude or has no color component at all. However, there are circumstances in which it is desirable to permit a viewer to manually adjust the threshold point at which the color killer operates, or to automatically alter the killer threshold point when the broadcast signal undergoes a predetermined change.
As attractive as it might be to include a viewer-operable or automatic color killer adjustment and means for selectively enabling and disabling automatic color level circuits, the inclusion of two such extra controls in the receiver adds unduly to the expense of the receiver.